1. Field of the Invention
The present invention relates to an optical pickup apparatus that includes a first laser unit including a first laser light source configured to emit a laser light of a first wavelength and a second laser unit including second and third laser light sources configured to respectively emit laser lights of second and third wavelengths in which the first laser unit and the second laser unit are disposed on different light paths by a beam splitter, and laser lights of the first, second, and third wavelengths selectively emitted from the respective laser light sources of the first laser unit and the second laser unit are guided onto a common light path through the beam splitter and guided to the respective sorts of optical recording mediums suitable for the laser lights, and, more particularly, to an optical pickup apparatus that divides each of the laser lights of the first, second, and third wavelengths by a diffraction grating into a main beam as well as preceding and subsequent sub-beams at the front and the back of the main beam.
2. Description of the Related Art
In an optical pickup apparatus used for optically recording/reproducing signals with the use of laser lights to an optical recording medium such as optical discs including DVD (digital versatile disc) and CD (compact disc), there is known an optical pickup apparatus which is compatible with DVD and CD with different recording densities with a single optical pickup apparatus.
Such an optical pickup apparatus compatible with DVD and CD includes laser light sources emitting a laser light of 645 nm to 675 nm in a red wavelength band suitable for DVD and also emitting a laser light of 765 nm to 805 nm in an infrared wavelength band suitable for CD, and the laser light to be used is switched according to an optical disc.
The laser light source generally include a semiconductor laser of a laser diode, and a multi laser unit is increasingly employed in which the laser light sources emitting the laser lights of the respective wavelengths includes a single laser chip, or laser light sources include respective laser chips and are installed on the same semiconductor base so as to be incorporated in a single package.
Also, in the optical pickup apparatus compatible with DVD and CD, an objective lens in which an annular diffraction grating is formed on an incident face is used, and a quality of each of the laser lights applied to each optical disc is ensured by correcting spherical aberration with respect to each of the optical discs of DVD and CD by diffracting each of the laser lights having wavelengths suitable for each of the optical discs with this diffraction grating so as to correspond to DVD and CD by a single objective lens.
The optical pickup apparatus compatible with DVD and CD has its optical path simplified by employing the above-mentioned multi laser unit and the single objective lens.
On the other hand, there is present a BD compatible optical pickup apparatus compatible with a Blu-ray Disc (trademark) (abbreviated as BD) standard using a laser light of 395 nm to 420 nm (e.g., 405 nm) in a blue-violet wavelength band.
Such a BD compatible optical pickup apparatus includes DVD and CD optical systems along with a BD reproduction (and recording) optical system since a BD apparatus supports reproduction (and recording) of DVD and CD.
Therefore, since a plurality of parts of each optical system is incorporated in the BD compatible optical pickup apparatus, it is preferred that the DVD and CD optical systems are configured to be simplified or compactified.
By the way, in an optical pickup apparatus, three beams, i.e., 0th order diffraction light beam and ±1st order diffraction light beams are formed by diffracting a laser light generally emitted from a laser unit with a diffraction grating, and tracking control is performed so that the main beam of the 0th order diffraction light beam is followed by a signal track of an optical disc by using the three beams.
A differential push-pull method is often employed as the tracking control method using three beams, and a current mainstream differential push-pull method is a single-line arrangement differential push-pull method using three beams of a main beam and preceding and subsequent sub-beams arranged in a single line on the same signal track to generate a tracking error signal such that an amplitude of a favorable tracking error signal can be acquired without the effect of a difference in pitch of signal track between optical disc types (see Japanese Patent Application Laid-Open Publication No. 9-81942).
This single-line arrangement push-pull method is achieved by providing a diffraction grating with a phase shift area that generates a pi-radian phase shift in a predetermined portion of the laser light emitted from the laser light source and by generating a pi-radian phase shift in a light-receiving spot of each of preceding and subsequent sub-beams received by a photodetector.
To simplify an optical path in an optical pickup apparatus compatible with three-wavelength which is compatible with BD, DVD, and CD, it is conceivable to use a multi-laser unit compatible with three-wavelength which is capable of emitting three laser lights in the wavelength bands adapted to BD, DVD, and CD and to employ a single objective lens compatible with the wavelength bands of BD, DVD, and CD. However, in the case of the multi-laser unit compatible with three-wavelength, because of an inability to satisfy the optical performance required from a relationship between dispersion and an optical axis of the laser light source for the laser light of each of the wavelengths, or due to quality, reliability, or cost, the multi-laser unit compatible with three-wavelength is not currently employed.
Therefore, it is practical that the optical pickup apparatus compatible with the three-wavelength comprises a two-wavelength laser unit compatible with DVD and CD and a BD-dedicated laser unit compatible with BD.
It is an object of the present invention to provide an optical pickup apparatus that achieves simplification of optical systems when one multi-laser unit of at least two laser units is used.